Using polarographic electrodes, oxygen tensions were measured in the cerebral cortex of gerbils subjected to repeated ischemias of short duration. In controls before ischemia, the cortex p02 values were found to vary from about 55 to zero tor. These variations did not seem to correlate with depth or topographical location. When this data was assembled into a bar histogram of frequency of particular 02 tensions, the distribution was gaussian shaped with its mean in the 20-25 tor interval. When the animal was made ischemic by bilateral carotid artery occlusion, the p02 dropped rapidly to zero, reaching this value in about 3 seconds. The dynamics of the recovery of the p02 was dependent on the duration of ischemia. Very short duration ischemias of a few seconds were followed by immediate recovery of p02 in a few seconds. Longer durations of ischemia (up to one minute) the p02 level recovers promptly with some overshooting. The p02 level rises above the pre-ischemic level by as much as two fold. With still longer durations of ischemia, (1 minute to 5 minutes), the p02 level rises sluggishly and often a delay in any observable sign of p02 recovery. Overshoot of p02 values above the preischemic values are not seen.